1. Field of the Invention
The present invention relates to an optical multiplexing communication system which multiplexes a plurality of optical signals to transmit. In particular, the present invention relates to a system and method for suppressing an effect of a usually-used channel of large optical power on a channel of weak optical power. For example, the present invention is applicable to a system in which quantum communications and usually-used optical communications are performed, and is particularly applicable to a system which multiplexes a quantum channel and a classical channel that are used for a quantum key distribution system.
2. Description of the Related Art
In the field of quantum cryptography, it is known based on Heisenberg's uncertainty principle that eavesdropping between a sender and a receiver can be detected with high probability. In other words, this fact indicates that a secret bit string (cryptographic key) can be shared between the sender and receiver without being eavesdropped. As an example of a procedure to share the secret information, BB84 (Bennett Brassard 84) protocol using four quantum states is known. A high level of security can be achieved by using a bit string created through this procedure as a key of Vernam cipher, which has been proved to be absolutely secure.
There have been proposed some quantum key distribution systems employing such a scheme. In particular, “Plug & Play” schemes proposed by the groups at the University of Geneva, Switzerland, are supposed to be promising schemes to bring a quantum key distribution system, which is sensitive to polarization, into practical use because the “Plug & Play” schemes can compensate for fluctuations in polarization occurring over an optical fiber transmission line. (See the followings:    G. Ribordy, J. D. Gautier, N. Gisin, O. Guinnard, and H. Zbinden “Automated ‘plug & play’ quantum key distribution” ELECTRONICS LETTERS, Vol. 34, No. 22 (Oct. 29, 1998), pp. 2116 to 2117;    A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden, and N. Gisin “‘Plug & Play’ systems for quantum cryptography” Applied Physics Letters, Vol. 70, No. 7 (Feb. 17, 1997), pp. 793 to 795; and    H. Zbinden, J. D. Gautier, N. Gisin, B. Huttner, A. Muller, and W. Tittel “Interferometry with Faraday mirrors for quantum cryptography” ELECTRONICS LETTERS, Vol. 33, No. 7 (Mar. 27, 1997), pp. 586 to 588.)A general configuration of a “Plug & Play” system is shown in FIG. 1.
In this plug & play system, optical pulse P is first generated by a laser LD in a device on the quantum key-receiving side (traditionally referred to as “Bob”) and then split into two pulses. One of the pulses, optical pulse P1, goes along a short path, and the other, optical pulse P2, goes along a long path, whereby the two pulses are sent to a sending-side device (traditionally referred to as “Alice”) with a small time delay between them. Upon receiving the optical pulses P1 and P2 sequentially, Alice allows the optical pulse P1 to be reflected by Faraday mirrors to make its polarization state rotate by 90 degrees and sends the optical pulse P1 back to Bob. Moreover, Alice similarly allows the optical pulse P2 to be reflected by the Faraday mirrors while modulating the phase of the optical pulse P2. Then, Alice sends phase-modulated optical pulse P2*a back to Bob. At Bob, the optical pulse P1 received from Alice passes along the long path, which is a different path from the path used when the optical pulse P1 was sent out. At the same time, Bob modulates the phase of the optical pulse P1 to obtain phase-modulated optical pulse P1*b. Meanwhile, the optical pulse P2*a, which has been phase-modulated on Alice's side, passes through the short path, which is a different path from the path used when it (i.e., the optical pulse P2) was sent out. Thereafter, the optical pulse P2*a interferes with the optical pulse P1*b phase-modulated on Bob's side. The result of the interference is detected by any one of photo detectors APD1 and APD2 (APD: Avalanche PhotoDiode). As a whole, the optical pulses P1 and P2, obtained by splitting the optical pulse P into two, follow the same optical path and then interfere with each other. Accordingly, since variations in delay due to the optical fiber transmission line cancel out, the result of the interference observed by the photo detector depends on a difference between the phase modulation on Alice's side and the phase modulation on Bob's side.
The “Plug & Play” system having such a configuration requires synchronization as described below:
1) On Alice's side, to modulate the optical pulse P2 received from Bob, the modulation operation should be made to follow the variations in delay due to the optical fiber transmission line;
2) On Bob's side, to modulate the optical pulse P1 reflected from Alice, the modulation operation should be made to follow the variations in delay due to the optical fiber transmission line; and
3) On Bob's side, when an optical pulse is received from Alice, a bias should be applied to the photo detectors in accordance with reception timing of the optical pulse (ultra-high-sensitivity reception in Geiger mode).
Moreover, as is well known, in the quantum key distribution systems, it is also required to establish sharing or synchronization in unit of bits for key data sharing as in the BB84 protocol.
In the quantum key distribution systems, however, unlike classical optical communications, its optical power is very small, at a single-photon level at most. Therefore, it is impossible to perform clock extraction from a quantum channel, as conventionally performed by using a classical channel. Here, the quantum channel is a communication channel in a state where the optical power of transmission from a sender to a receiver is very weak, at most one photon per bit, whereas the classical channel is a communication channel in the range of usually-used optical power or a multi-photon communication channel.
Specifically, when communication is performed using the quantum channel with light at a very low optical power level, the quantum efficiency of the APD (photo detector) is small. Therefore, for example, even if a sender sends data with a mark ratio of ½, the mark ratio becomes far smaller than ½ at a receiver. Consequently, data losses occur, and an accurate-period clock signal cannot be extracted. The classical channel is therefore generally used to provide synchronization for such a quantum channel.
For example, Japanese Patent Application Unexamined Publication No. H08-505019 proposes a method using a classical channel to provide bit synchronization, frame synchronization and other system calibration. According to this method, both a quantum channel and a classical channel are set on the same transmission line, and the classical channel is used to perform clock synchronization for the quantum channel where the optical power is very weak.
As another example of the quantum and classical channels being set on the same transmission line, Japanese Patent Application Unexamined Publication No. 2003-37559 discloses a signal state control device. According to this conventional example, a quantum channel and a classical channel are multiplexed on the same transmission line. The polarization state of signal light on the quantum channel is controlled in real time by monitoring check light on the classical channel.
Furthermore, there has been also proposed a technology taking into consideration the influence between the quantum channel and classical channel multiplexed on the same transmission line. (See M. S. Goodman, P. Toliver, R. J. Runser, T. E. Chapuran, J. Jackel, R. J. Hughes, C. G. Peterson, K. McCabe, J. E. Nordholt, K. Tyagi, P. Hiskett, S. McNown, N. Nweke, J. T. Blake, L. Mercer, and H. Dardy, “Quantum Cryptography for Optical Networks: A Systems Perspective” LEOS 2003, Vol. QE-14, pp. 1040 to 1041.) Goodman et al. discloses a system in which an attenuator is arranged on Alice's side to adjust the power level of a classical wavelength multiplex (DWDM) signal when the DWDM signal in the 1500 nm band and a quantum key distribution (QKD) signal in the 1300 nm band are transmitted through a single common optical fiber, and in which the DWDM signal and QKD signal are combined by a 10/90 coupler. Further, another system is also described in which the power of noise in the 1300 nm band, which arises from the DWDM signal and affects the QKD signal, is effectively suppressed by replacing the attenuator and coupler with a band multiplexer.
However, in the configuration in which the quantum channel and classical channel are transmitted on the same transmission line by means of, for example, wavelength division multiplexing (WDM) as described in Japanese Patent Application Unexamined Publication Nos. H08-505019 and 2003-37559, crosstalk between the channels exists in practice. For example, crosstalk is caused by spontaneous emission light from a laser light source (laser diode) and by nonlinear optical effects (such as Raman scattering and parametric amplification).
In a usually-used optical communication system, since signal optical power on each channel is equal to that on another, the power of crosstalk light does not exceed the power of main signal light. In a quantum key distribution system, however, since the optical power on a quantum channel is very weak, the power of crosstalk light from an adjacent classical channel exceeds the power of a signal on the quantum channel. Accordingly, the crosstalk light becomes noise light to the quantum channel, degrading the signal-to-noise ratio of the quantum channel.
Moreover, in a usually-used optical communication system, it is not required to pay attention to crosstalk at the time of wavelength multiplexing for WDM transmission. In a quantum key distribution system, however, care needs to be taken at the time of multiplexing so that the power of crosstalk light from the classical channel does not exceed the power of a signal on the quantum channel.
For example, the Goodman et al. proposes a method of reducing the influence of crosstalk by employing a widened channel spacing between the quantum and classical channels in the 1300 nm and 1500 nm bands, as well as a method of suppressing the power of noise due to the spontaneous emission light from the classical channel toward the quantum channel by disposing the attenuator and 10/90 coupler, or the band multiplexer, on Alice's side.
However, according to these conventional methods, EDFA (Erbium-Doped Fiber Amplifier) is provided to the classical channel on Alice's side. Therefore, spontaneous emission light arises not only from each laser for DWDM but also from the EDFA. Accordingly, crosstalk cannot be effectively eliminated unless a wide spacing is made between the quantum channel and classical channel.
Moreover, according to the Goodman et al., filtering is merely performed on Bob's side, and no consideration is given to the influence due to the nonlinear optical effects occurring when an optical signal is propagating along the transmission line. Accordingly, this conventional configuration cannot avoid the influence due to the nonlinear optical effects.
Furthermore, according to Goodman et al., the 1300 nm band is used for the QKD signal. Therefore, transmission loss is large, limiting the transmission distance. In addition, the 1300 nm or 1500 nm band is exclusively used by the quantum channel, resulting in inefficient use of frequencies and wavelength resources.